This invention relates to methods for fabricating micromechanical devices having microstructures suspended above a substrate and, more particularly, to methods for releasing the suspended microstructures from contact with the substrate and other elements of the device.
Micromechanical sensors for sensing a physical quantity, such as acceleration, vibration or electrostatic potential, are useful in many applications, including but not limited to airbag deployment and active suspension in automobiles, and guidance systems in military weapons. A micromechanical sensing apparatus may include a micromechanical sensor in the form of a suspended microstructure and a circuit responsive to the micromechanical sensor for providing an output representative of a sensed quantity. The suspended microstructure includes stationary and movable elements which are conductive. The micromechanical sensor may be configured for sensing acceleration. When an acceleration sensor of this type is subjected to an accelerative force, the movable element moves relative to the stationary element, producing an output that is sensed by the circuit. The stationary and movable elements form a capacitor which changes in capacitance when the sensor is subjected to an accelerative force.
In fabricating suspended microstructures, a layer of material is typically deposited over a previously deposited sacrificial layer and then is etched into the desired form. The sacrificial layer is then removed by a wet etching process in which the wafer is exposed to a chemical etching solution that dissolves the sacrificial layer but does not affect the material from which the microstructure is formed. The wafer is then washed in a rinse fluid. As the rinse fluid is removed, the surface tension of the liquid exerts a force on the delicately suspended microstructure, tending to pull the microstructure into contact with the substrate or with other portions of the microstructure. A combination of forces, including adhesive forces and electrostatic forces, makes it difficult to separate the contacting portions. Electrostatic force may also contribute to the initial attraction of the microstructure to the other surfaces. Such contact between the suspended microstructures and the other elements of the device must either be avoided or repaired. Otherwise, the micromechanical device is unusable and must be discarded.
A technique for supporting the microstructures during wet etch is disclosed in U.S. Pat. No. 5,314,572 issued May 24, 1994 to Core et al. Photoresist pedestals are inserted in the sacrificial layer between the suspended microstructure and the substrate, and photoresist spacers are inserted in the microstructure layer between non-contacting portions of the suspended microstructure, so that the photoresist pedestals and spacers support the microstructure during the wet etching and drying process used to remove the sacrificial layer. A dry etch is used to remove the support structures. While this approach provides satisfactory results, it involves additional process steps and cost.
Other techniques for unsticking microstructures in micromechanical devices have been proposed in the prior art. A technique which involves the use of a low surface tension liquid with a surfactant or a supercritical fluid is disclosed in U.S. Pat. No. 5,482,564 issued Jan. 9, 1996 to Douglas. A technique which involves irradiation of the device with a set of microwave frequencies is disclosed in U.S. Pat. No. 5,412,186 issued May 2, 1995 to Gale. A technique for repositioning mirror elements of a digital micromirror device by irradiation with a short high energy pulse of visible light is disclosed in U.S. Pat. No. 5,717,513 issued Feb. 10, 1998 to Weaver.
All of the known techniques for releasing suspended microstructures from contact with other elements of the device have had one or more disadvantages, including added cost and complexity, and lack of effectiveness under certain conditions. Accordingly, there is a need for new and improved methods for releasing suspended microstructures from contact with the substrate and other elements of a micromechanical device.
According to a first aspect of the invention, a method is provided for releasing a structure from contact with a substrate in a micromechanical device. The method comprises the step of irradiating the structure with energy having parameters selected to produce in the structure a thermal gradient normal to the surface of the structure which causes upward bowing and release of the structure from the substrate.
The energy parameters may be selected such that the optical absorption depth of the energy in the structure is about one-third to one-half the thickness of the structure. Preferably, the structure is irradiated with laser energy and, more preferably, the structure is irradiated with pulsed laser energy. The pulsed laser energy may comprise one or more laser pulses applied to the structure.
The temperature gradient produced by the pulsed laser energy creates a strain gradient, due to thermal expansion, which causes the structure to bow upwardly. The structure remains bowed upwardly and free of surface adhesion until the thermal gradient from top to bottom disappears. During the time that the structure is bowed upwardly, the microstructure supports react and continue to hold the structure up.
According to another aspect of the invention, a method is provided for releasing a structure from contact with a substrate in a micromechanical device. The method comprises the steps of selecting energy parameters to produce in the structure a thermal gradient normal to the surface of the structure sufficient to cause upward bowing of the structure, and irradiating the structure with energy having the selected energy parameters, wherein the structure is released from the substrate.
According to a further aspect of the invention, a method is provided for releasing a polysilicon structure from contact with a substrate in a monolithic micromechanical accelerometer. The method comprises the steps of selecting laser energy parameters to produce in the polysilicon structure a thermal gradient normal to the surface of the polysilicon structure sufficient to cause upward bowing of the polysilicon structure, and irradiating the polysilicon structure with laser energy having the selected laser energy parameters, wherein the polysilicon structure is released from the substrate.
In a preferred embodiment, a pulsed laser at a wavelength of 532 nanometers is used for releasing the polysilicon structure from contact with the substrate. The pulsed laser energy may have a pulse width of 5-7 nanoseconds and an average energy density at the polysilicon structure of about 40 to 120 millijoules/square centimeter.